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The effects of ocean acidification and temperature change on the West Coast rock lobster (Jasus lalandii)

Knapp, Jarred Lee (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The West Coast rock lobster (WCRL), Jasus lalandii, is a critical marine fisheries resource for South Africa and may in future be negatively affected by the changes in seawater parameters associated with the ongoing anthropogenic carbon dioxide (CO2) emissions. These CO2 emissions have been linked to a global decrease in ocean pH (termed “ocean acidification”) and an increase in temperature. There are strong estimates that these changes are to worsen in coming centuries. This warranted research because of 1) the low current level of the resource (2.6% of pristine) and 2) the relatively unexplored physiological- and other biological responses of the WCRL to environmental stressors. This information is essential for the sustainable management of the resource by government scientists in times of global- and regional climate change.
In the short term, it was found that the WCRL was able to rapidly and reversibly respond to acute changes in seawater pH (pH 7.4), this was achieved primarily through the active up-regulation of bicarbonate levels in the haemolymph. Maintaining extracellular pH protects oxygen transport mechanisms, which are sensitive to pH changes due to the large Bohr effect that this study also revealed, in the respiratory protein, haemocyanin of adult WCRL.
The energy cost of actively maintaining extracellular pH, however, is expected to affect growth and potentially survival in the long term. This was tested on juvenile WCRL that were exposed to a reduced seawater pH of 7.3 (18.8 °C) over a period of 28 weeks. Results revealed that survival was not influenced and acid-base regulation in the hypercapnia-exposed lobsters was maintained throughout the duration of the trial, however, this led to a reduced growth rate. Subsequently, in order to replicate field conditions more closely, a combination of effects, namely seawater pCO2 (pH 8 and 7.3) and different temperatures (15.6 and 19 °C) on the growth of juvenile WCRL were assessed over an exposure period of 48 weeks in a second chronic trial. In contrast to the initial trial (28 weeks), where hypercapnia was assessed separately, lobsters exposed to hypercapnia had a higher growth rate than those at the same temperature exposed to a “natural” (normocapnic) seawater pH. The difference was interpreted as an indication that food availability/quality may negatively affect stress response, as feeding in the first trial was later considered “sub-optimal” in comparison to that of the second trial. In the latter, although both hypercapnia and temperature affected growth rates, temperature was the largest contributor to differences observed between treatments. The order of growth rates for lobsters from different treatments was: hypercapnia/high temperature > normocapnia/high temperature > hypercapnia/low temperature > normocapnia/low temperature. In this trial too, irrespective of treatment, lobsters were able to maintain extracellular pH within a relatively narrow range over the extent of the trial and survival was not negatively affected by hypercapnia or high temperature.
In order to compare the sensitivity of juvenile WCRL to that of adults, with regards to the effect of changes in extracellular pH on oxygen transport, and to assess the impact of chronic hypercapnia, haemocyanin from juveniles was studied in detail after the first growth trial. This revealed that juvenile WCRL have a similar Bohr effect to that of adults. In addition, the haemocyanin of hypercapnia-exposed juveniles showed an increased affinity to oxygen caused by an intrinsic change in its molecular structure. This was interpreted as an energy-saving mechanism, because at the same time, haemocyanin concentration in these animals was lower than in normocapnic lobsters.
At the termination of the second chronic trial, the immunological response to the combined stressors was assessed, namely total circulating haemocyte counts (THC) and the ability to clear/inactivate an introduced dose of a bacterium, Vibrio anguillarum. A pilot experiment on non-treated juveniles revealed a similar resting THC to that of other lobster species, and culturable V. anguillarum was rapidly cleared from their haemolymph. The effect of chronic exposure to a combination of effects, namely hypercapnia and different temperatures, was subsequently tested after termination of the second chronic trial. There were no differences between treatments in a) baseline THC (i.e. before bacterial challenge) and 2) the capability to clear culturable bacteria from haemolymph. The only difference was the circulating THCs post-bacterial challenge, as they were reduced in the hypercapnic-, high temperature treatment, compared with all other treatments. The reason is unknown, but it is speculated that it may have been linked to an increased metabolic demand in these lobsters.
Overall, these results demonstrate the great plasticity of the WCRL at the molecular-, biochemical and physiological level. They provide important initial information for government fisheries scientists to aid in predicting future development of, and potential threats to the WCRL resource, as well as providing a platform from which the direction of future studies can be determined.